In cellular mobile radio systems a communication connection is established between a mobile radio device, generally also referred to as a terminal, a mobile terminal or user equipment (UE), and the mobile radio network via a so-called base station. The base station serves mobile radio users in a specific area a so-called cell via one or more radio channels. Such a base station also referred to as node B in the UMTS standard provides the actual radio interface between the mobile radio network and the mobile terminal. It deals with the radio operation with the different mobile users within their cell and monitors the physical radio connections. It also transmits network and status messages to the terminals. A distinction is made between two connection directions in mobile radio. The downlink DL describes the direction from the base station to the terminal, the uplink UL the direction from the terminal to the base station.
Generally a number of different transmission channels exist in each direction. There are therefore so-called dedicated channels, for example, for the specific transmission of information from or for a specific terminal. There are also so-called common channels, which serve to transmit information intended for a number or even all of the terminals from the base station. Similarly there are common channels in the backward direction, which the different terminals share, for example, for the transmission of short messages or control data to the base station, each terminal only using the channel for a short time. The different channels are thereby generally multi-layer in structure. The base is a so-called physical channel, referred to as layer 1 for example in the UMTS standard.
To transmit the different data elements, different logical channels are implemented on different higher layers on top of the physical channel, i.e., the lowest layer. Data is thereby generally transmitted on the physical channel in a packet-oriented manner, i.e., the data to be transmitted is divided into individual packets, which are sent temporally one after the other. Control data is also transmitted in packet form generally with a temporal offset parallel to the useful data to be transmitted. This is required on the recipient side in order to identify the packets and re-assemble them correctly. The control data can thereby include a packet number for example, which serves to identify a data packet.
A typical example of such a physical channel, on which such a transmission method is used, is the so-called HSDPA (High Speed Downlink Package Access) channel. This is a downlink channel according to the most recent UMTS standard. The transmission method used there is a fast, so-called HARQ
Method (HARQ=Hybrid Automatic Repeat Request). An ARQ method (Automatic Repeat Request) is an error protection method, with which the blocks to be transmitted are continuously numbered and provided with a block check sequence, which the recipient uses to decide whether a transmission error has occurred. Correct blocks are acknowledged by the recipient using a so-called ACK signal. The recipient responds to an incorrect block either with a negative acknowledgement, a so-called NACK signal, or the block is ignored, whereupon the sender repeats the transmission after a predefined period of time. The sender only sends a new packet on the same channel, when the immediately preceding block has been positively acknowledged by the recipient (so-called stop and wait method).
The term hybrid means that parity bits (check bits) are also transmitted for error protection purposes. A multi-channel stop and wait protocol (so-called n-channel stop and wait) is used on the HSDPA channel. Temporal distribution is thereby used on the physical channel to implement a number of time channels, to which different transmission time intervals are assigned, each corresponding to a block length. This allows further blocks to be sent in the other time channels while waiting for the acknowledgement for a transmitted block in one time channel. The channel number of the respective time channel must be sent specifically as a control parameter to the recipient from the sender for example. Whether a transmitted block is a new packet or a re-transmission of the last packet can be seen from the packet number referred to above for identifying the data packet.
Only a limited number of packet numbers are thereby available for each time channel and these are always used in a cyclically alternating manner. In other words, after the last packet number has been used, the next new data packet is given the first number again, etc. In the case of the HSDPA channel, this packet number is referred to as the so-called new data indicator NDI. In the case of the HSDPA, only 1 bit is made available for this and it changes value with each new packet.
The different control parameters required for control purposes, e.g., the channel number and packet number, first have to be coded in the context of a source-coding before transmission. In the case of the HSDPA, the channel number is source-coded into 3 bits. The packet number is thereby source-coded separately into a 1-bit packet number. The information data thus generated is then channel-coded. In a so-called rate matching method, this data is then reduced such that it can be transmitted within a defined transmission time interval of a time channel, which is two milliseconds in the case of the HSDPA.
However the use of this HSDPA method for uplink signaling from the individual terminals to the base station is relatively unfavorable. A so-called SHO method (SHO=soft handover) method is frequently used on the uplink channels. With this method, a radio connection is simultaneously maintained between the terminal and the network in a parallel manner via a number of base stations, such that a terminal moving within the network can be handed over in a smooth manner between the individual base stations. In the SHO method, the power regulation of the terminal is controlled such that decoding can take place successfully on one of the connections at least. This means, however, that often only the base station with the best channel conditions can decode the associated control data. For other base stations using the SHO method, a number of packets with the associated control data may not be comprehensible. Also a soft-combining method is generally used to improve transmission quality with the current standards. Different transmissions of a packet are thereby superimposed before decoding, i.e., the first transmission is used first and if this cannot be decoded, the re-transmission is superimposed with the first transmission, thereby increasing the signal energy of the packet. With this per se advantageous combination of soft handover and soft combining the problem arises that a 1-bit packet number is not adequate to prevent incorrect superimposition of different packets.
This is illustrated using the following example: Where there are three successive packets, if only one 1-bit packet number is used, these are given the packet number 0,1,0. If one of the base stations using the SHO method does not receive the middle packet with the packet number 1, but another base station does, the receiving base station will acknowledge the packet. The terminal then sends the third packet again with the number 0. The base station which could not decode the middle packet then assumes that the third packet is a re-transmission of the first, since the packet number has not changed between two decoded packets. This base station will, therefore, try to decode the packet with the transmissions of both packets superimposed. As the packets do not, however, correspond, this decoding attempt will inevitably fail. Such frequently occurring events have a negative effect on system performance.
One possible way of avoiding this problem would be to use an n-bit packet number where n>1. In this instance, there is only a risk of confusing a new packet and a re-transmission of the last packet, if the recipient in question has in the meantime been unable to decode any of the transmissions of 2n−1 packets in sequence. One disadvantage of this method is that n-bit signaling outlay results, which is only required in instances where an SHO situation actually exists. This is the case during approximately 30% of the transmission time. In 70% of the transmission time n−1 bit is in principle unnecessary and simply increases the signaling outlay.
Signaling outlay can be saved if an HARQ method is used, in which a quite specific time slot is available in each HARQ channel from a fixed time. This has the advantage that the HARQ channel number does not have to be sent specifically and can be determined for example from the so-called system frame number SFN. However, it has the disadvantage of reduced flexibility with regard to resource allocation, with the result that the system as a whole cannot be used optimally and packet transmission is delayed further. As a result, the method as a whole is less efficient.
It would also be possible, when using the SHO method, not to implement soft-combining and not to superimpose re-transmissions. Since the packets are not superimposed, it is not necessary to transmit a packet number on the physical channel and the signaling for this is not required. One disadvantage of this is, however, that the gain due to the soft-combining method is lost and the data throughput as a whole is reduced.